Ionic liquids are in the liquid state at room temperature range (generally below 100° C.), which consist of organic cations and inorganic anions (or organic anions). Ionic liquids which are regulable have been recognized as designer solvents, because ionic structure may be changed as needed by designer. The first ionic liquid, ethylamine nitrate was discovered as early as 1914. However, development in this field has been slowing down after the first discovery. Until 1992, when Wikes's team successfully synthesized 1-ethyl-3-methyl imidazolium tetrafluoroborates ionic liquid ([EMIM]BF4) having low melting point, resistance to hydrolyzation, and good stability, studies on ionic liquids were developed rapidly. Since then a series of ionic liquids systems have been developed. Ionic liquids are primarily used for electrochemical investigation purpose. Recently, ionic liquids have drawn more public attention because ionic liquids can be used as green solvents for organic and macromolecular synthesis.
Compared to conventional organic solvent and electrolyte, ionic liquids exhibit a range of advantageous properties such as (1) having a melting point below or near room temperature, being liquid in a wide temperature range; (2) having a low vapor pressure, being non-volatile, colorless and odorless; (3) having a wide and stable temperature range, good chemical stability and wide electrochemical potential window range; (4) having a excellent solubility, can dissolve lots of inorganics and organics (5) having no ignition point, and being nonflammable; (6) can be recycled, environmentally friendly. So ionic liquids are promising electrolytes for double-layer capacitors.
Traditional electrolytes are prone to decompose at high voltage, causing a sharp rise of internal resistance and a rapid fall of capacitance. Therefore stability of electrolyte has always been the critical factor in specific energy of capacitor.